How do I check if my client has viewed the quote/invoice I’ve sent him?

You can simply click on the particular quote or invoice and check if it has been viewed by the client. A tiny eye icon on the right corner accompanying the word Viewed suggests that the client has seen it.

Alternatively, go to the quote or invoice tab under Sales on the home page of Zoho Books. Click on the drop down next to All Quotes or All Invoices and select Client Viewed. You can now see only the client viewed quotes/invoices.